maybe someone else can clarify; these recent papers suggest a surprising turnaround in the quantization of General Relativity, contrary to some earlier papers by other people, they predict no quantum gravity dispersion in longrange transmission of light: On low energy quantum gravity induced effects on the propagation of light---Gleiser/Kozameh/Parisi http://arxiv.org/gr-qc/0304048 Lorentz Invariance and the semiclassical approximation of loop quantum gravity---Kozameh/Parisi http://arxiv.org/gr-qc/0310014 the first of these two papers is also available in the journal "Classical and Quantum Gravity" vol. 20 pp. 4375-4385 the second has just been posted as pre-print this month on page 12, in the conclusions, the first paper says "In Section 3 we show that a very natural assumption leads to Lorentz invariant field equations. Our conclusion is that we have no reason to believe that a quantum theory of gravity would change the invariance..." on page 2, in the introduction, the second paper says "In recent years there as been hope of observing quantum gravity effects via the propagation of light through cosmological distances. This hope is based in some models describing the interaction of quantum Maxwell and gravitational fields that predict a breakdown of Lorentz invariance at a linearized level in the semiclassical approximation. The common feature in these models is a non standard dispersion relation which shows that spacetime behaves as a medium with a frequency dependent index of refraction..." Here they cite papers by Ellis/Amelino-Camelia etal., Gambini/Pullin, Alfaro/Morales-Tecotl/Urrutia, and Sahlmann/Thiemann. their analysis appears to contradict results by Ellis and also by Sahlmann/Thiemann. It concludes that their supposed predictions are wrong and a dispersion relation (at least of the kind earlier discussed) is not to be expected my take on it: theory is supposed to develop guided by observation and experiment. LQG is close enough to conventional GR that people can try out details of the theory and various modifications and crank out predictions and see if they are right. Apparently Sahlmann/Thiemann analysed photon/graviton interaction in some fashion and got some numbers and they didnt match observation and these people in Argentina are setting it up slightly differently and saying that they, not Sahlmann/Thiemann and not John Ellis etal., are doing the analysis right and are in agreement with the observations. I guess I'll have to wait and see if other people think the work of these people at the University of Cordoba is sound, and whether they cite it and so forth. As a general thing it seems like its good its happening.